June 21st or 22nd is the summer solstice day every year. On the summer solstice, the sun's direct position on the ground reaches the northernmost point of the year, almost directly to the Tropic of Cancer, and the daytime in the northern hemisphere reaches the longest, and the farther north it is, the longer it is.

The summer solstice is the longest day in a year in the northern hemisphere, and it takes about 14 hours from sunrise to sunset in southern China. Although the summer solstice has the longest day and the highest sun angle, it is not the hottest time of the year. Because, when the heat near the surface continues to accumulate and does not reach the maximum, the real hot weather is calculated based on the summer solstice and beginning of autumn.